The protocol is designed to evaluate children with cutaneous or systemic mastocytosis. Patients with all variants of pediatric-onset mastocytosis: urticaria pigmentosa, diffuse cutaneous mastocytosis, mastocytoma, and indolent systemic mastocytosis, are being studied. The information we are collecting from subjects enrolled in the protocol provides the much needed information required to identify factors that may be associated with disease that does not resolve. Moreover, in those children with indolent systemic mastocytosis, this protocol will identify prognostic markers for disease progression into adulthood. In our recent studies, we have thus found serum tryptase to reflect extent of disease and to serve as an important biomarker of both disease regression and progression. We have also identified unique bone marrow biopsy findings that may similarly be used to predict disease progression and may lead the way to an improved classification of pediatric onset mastocytosis.